When drilling a hydrocarbon recovery well, the drill string is conventionally rotated to drive the drill bit. When a mud motor is used along the drill string to rotate the bit, the drill string is frequently rotated at a speed less than the bit to drill a straight section of a well. Drilling mud is thus circulated through the drill string and to the bit, and up the annulus between the drill string and the wellbore. As one or more joints are added to the string (running pipe), the drilling mud circulation process is briefly stopped to make up the threaded interconnection of the new joint to the drill string. Although the drill string is normally full of fluid, fluid pressure supplied by the mud pumps is lost when the pumps are shut off as the threaded connection is being made up. Restarting circulation can be difficult, and may lead to numerous drilling and well control problems. The process is reversed when pulling drill pipe from a well.
The prior art includes methods designed to allow a drill joint to be added to or removed from a drill string while circulation of mud continues. One prior art technique utilizes a coupler for substantially surrounding the threaded connection. Various rams, preventers, slips, and other closing devices substantially seal fluid within the coupler as the connection is made up. Devices of this type are disclosed in U.S. Pat. Nos. 6,119,772, 6,591,916, 6,739,397, 7,028,586, and 7,252,151. Publication WO98/16716 also discloses a continuous circulation drilling method. SPE 98947 by Torsvoll et al. discusses a continuous circulation valve for use while drilling. U.S. Pat. No. 4,478,244 discloses a mud saver valve which may be threaded to a drill string to reduce mud spills. U.S. application Ser. No. 11/786,495 discloses a safety valve with a ball rotatable engaging an actuator sleeve.
There are significant disadvantages to the above technique. First, the mechanisms for accomplishing continuous circulation by enclosing the connection involve expensive couplers, and numerous sealing devices are provided to minimize leakage of mud from these couplers. The device is also expensive, and at least in some applications slow down the makeup or breakout process, thereby contributing to higher drilling costs. Often techniques are complicated to use at a drilling rig, and/or slow down the time required to make up or break apart a threaded connection.
The disadvantages of the prior art are overcome by the present invention, in an improved technique for continuously circulating drilling mud in a drill string is hereinafter disclosed.